public class RelationshipClass : Dataset, System.IDisposable  

public async Task JoiningFileGeodatabaseFeatureClassToOracleQueryLayer()
{
    await ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
    {
        using (Geodatabase geodatabase = new Geodatabase(new FileGeodatabaseConnectionPath(new Uri("C:\\Data\\LocalGovernment.gdb"))))
        using (Database database = new Database(new DatabaseConnectionProperties(EnterpriseDatabaseType.Oracle)
        {
            AuthenticationMode = AuthenticationMode.DBMS,
            Instance = "instance",
            User = "user",
            Password = "password",
            Database = "database"
        }))

        using (FeatureClass leftFeatureClass = geodatabase.OpenDataset<FeatureClass>("Hospital"))
        using (Table rightTable = database.OpenTable(database.GetQueryDescription("FacilitySite")))
        {
            Field originPrimaryKey = leftFeatureClass.GetDefinition().GetFields().FirstOrDefault(field => field.Name.Equals("facilityId"));
            Field destinationForeignKey = rightTable.GetDefinition().GetFields().FirstOrDefault(field => field.Name.Equals("hospitalID"));

            VirtualRelationshipClassDescription description = new VirtualRelationshipClassDescription(
        originPrimaryKey, destinationForeignKey, RelationshipCardinality.OneToOne);

            using (RelationshipClass relationshipClass = leftFeatureClass.RelateTo(rightTable, description))
            {
                JoinDescription joinDescription = new JoinDescription(relationshipClass)
                {
                    JoinDirection = JoinDirection.Forward,
                    JoinType = JoinType.LeftOuterJoin
                };

                Join join = new Join(joinDescription);

                using (Table joinedTable = join.GetJoinedTable())
                {
                    // Perform operation on joined table.
                }
            }
        }
    });
}